Final answer:
DNA polymerase used in PCR must be thermally stable so that it remains active during the high-temperature cycles necessary for DNA denaturation and can function efficiently without being replenished after every cycle.
Step-by-step explanation:
The DNA polymerase used in the Polymerase Chain Reaction (PCR) must be thermally stable to withstand the high temperatures required for the denaturation step. The denaturation process involves heating the sample to around 95 °C to separate the DNA strands. A conventional DNA polymerase enzyme would denature at this temperature, just as proteins in an egg white coagulate when boiled.
This is why Taq DNA polymerase, derived from the thermophilic bacterium Thermus aquaticus, is used since it remains active even at high temperatures of up to 98 °C. The thermal stability of Taq polymerase is crucial because it allows the enzyme to remain functional through the multiple heating and cooling cycles of PCR, without the need to add fresh enzyme after every cycle. Using a thermally stable enzyme like Taq polymerase not only makes the PCR process more efficient but also ensures high fidelity in DNA replication.